Generally, a gas having high chemical reactivity is supplied to the process chamber used in the semiconductor manufacturing facilities, the chemicals manufacturing facilities and the like. Accordingly, the exhaustion system for the process chamber is required to exhaust high reactivity gases in safety and with a high degree of efficiency.
Therefore, to conduct an efficient exhaustion from the process chamber, it is necessary to employ an exhaustion pump with a high compression ratio, which can exhaust at a high exhaustion velocity (1/min) even if the suction pressure is low. However, in reality, the vacuum exhaustion pump having a high compression ratio is not easily available. Accordingly, it is necessary for the conventional exhaustion system for the process chamber to overcome two problems, one that the gas is to be exhausted with a high degree of efficiency using a pump having a relatively low compression ratio, and the other that the overload on the pump is to be avoided by keeping small the pressure difference between the primary side and the secondary side of the exhaustion system. To solve the two problems, the diameter of the pipings for the exhaustion is made large (a nominal diameter of approx. 4 inches) so that the conductance of the pipe passage is great. For the same reason, the valve having a large diameter is employed.
The fluid flow is classified into two regions, a viscous flow region and a molecular flow region in regard to the relationship between the pressure and the inside diameter of the flow passage. To conduct an efficient exhaustion, it is required that the exhaustion be conducted in the viscous flow region. To achieve the viscous flow region, the inside diameter D of the flow passage should be L≦D (where L: the mean free path of gas molecules and D: the inside diameter of the flow passage). Further, there exists the relationship, L=4.98×10−3/P, between the mean free path L and the pressure P.
The relationship between the pressure and the inside diameter to attain the viscous flow region inside the pipings is shown above. By raising the pressure higher, the mean free path L can be made smaller with the result that the inside diameter D of the pipings to attain the viscous flow region can be made small.
However, as stated above, since the conventional pump has a comparatively small compression ratio (approx. 10), it is not possible to raise the pressure on the discharge outlet side. For example, when the pressure on the chamber side (the primary side) is 10−3 Torr, the discharge outlet side pressure becomes as low as approx. 10−2 Torr. This means that the pipings having the inside diameter of 5 cm or larger is required to attain the viscous flow region with more certainty.
As a result, with the conventional vacuum exhaustion system, there is a problem that since the piping system having a large diameter has been required, the facility is forced to be large in size. At the same time, there is another problem that since a larger inside diameter of the vacuum piping system results in a larger volume inside the pipings, it takes a long time for the vacuum exhaustion. Furthermore, there is another problem that to conduct an efficient exhaustion in a short time with the small-sized vacuum exhaustion system, an expensive, high-performance vacuum pump having a large compression ratio and a high exhaustion velocity is needed.
On the other hand, with the vacuum exhaustion system, the dissociation (decomposition) of gases retained inside the pipings occurs when the vacuum pump is out of operation for a long time, thus causing the corrosions of the pipings resulting from the precipitation of substances produced by the decomposition inside the pipings. Particularly, when the substances, water and moisture produced by the dissociation of gases inside the pipings accumulate and adhere on the inside walls of the pipings and the piping parts of the valves, not only the afore-mentioned corrosion problem but also the cloggings and valve seat leakages occur.
If the piping system is heated, the risk of causing the corrosions and the like is reduced because there will be less chance of water and moisture adherence along with the temperature rise.
However, if the temperature is raised inside the pipings, the dissociation (decomposition) of gases is caused such that the substances produced by the decomposition deposit and accumulate inside the pipings, causing the corrosions, cloggings and valve seat leakages.